The present invention relates to an instrument and a method for measuring partial electrical discharges in an electrical system, the instrument comprising:                an input stage set up to receive an analogue signal representing one or more pulses of partial discharges to be measured and to generate at its output a digital representation of the entire wave form of said one or more pulses,        an output stage set up to transfer data in digital shape at the output from the instrument.        
The technical sector of the present invention is that of the diagnostics of electrical systems (in particular in high voltage), by the measurement/processing of partial electrical discharges and possibly other quantities.
It should be noted that a partial discharge is an electrical discharge that involves a limited portion of an insulator of an electrical system, therefore it does not cause the immediate failure of the system, but its progressive degradation. Thus, partial discharges have, for their nature, a development that is substantially limited to a defect of the insulating system. In this light, diagnostic techniques based on the measurement and interpretation of partial discharges are among the most promising and they are widely studied in scientific research, because the study of partial discharges allows to investigate the nature of the defects of the insulating system in which the discharges themselves reside.
However, the measurement together with the subsequent evaluation of partial discharges for diagnostic purposes has not yet completely been included among industrial standards, as an instrument to plan the maintenance and/or replacement of electrical components operating at high voltage, because of the difficulty that is encountered in interpreting the results of the measurements.
With regard to the measurement of partial discharges, various types of techniques have been developed, exploiting the different physical phenomena associated to the arrival of the discharges, such as optical, acoustic and electrical techniques. The present invention relates, in particular but not exclusively, to electrical measuring techniques, which consist, as is well known, of measuring the current pulses that travel through a measuring circuit coupled with the electrical system in question. Said measured current pulses (hereafter called discharge pulses, for the sake of simplicity), have a time development, which depends on the dynamics with which the partial discharges occur (i.e. on the physics of the discharge phenomena) and on the nature of the means that the measured pulses traverse, in their path from the discharge site (in which the discharges originate) to the measurement site. Thus, the time development of the discharge pulses, consisting of the wave form of the pulses themselves, comprises information that is precious from the diagnostic viewpoint, both with regard to the physical phenomena associated with the discharges (correlated to the nature of the defects of the insulating system), and the nature of the medium which the measured pulses traverse (correlated to the location of the defects within the insulating system).
With regard to the difficulties in the interpretation of the results of the measurements of partial discharges, they depend not only on the need to have available a specific experience and case record, but also on the fact that the measured data could be unreliable or not significant.
In this light, the problems that can compromise diagnostics through the evaluation of the measurements of partial discharges are essentially two:                in the measurement of the signals associated with the partial discharges, there is a loss of information that is essential for a subsequent evaluation of the signals for diagnostic purposes (a loss of information can be constituted, for example, by a failure to measure an impulse, or by a failure to measure the wave form of a pulse);        during said measurement, noise may be superposed to the discharge signals, or signals due to different sources may be mutually superposed, with a consequent objective difficulty in interpreting the results, given the impossibility to perform significant statistical processing on heterogeneous data and/or on data that are not pertinent to the individual phenomena to be evaluated.        
With regard to said loss of information while performing the measurement, it should be noted that the signals associated to the partial discharges are electrical impulses having a very high frequency content (they have up-ramps in the order of nanoseconds, or tens of nanoseconds) and, in certain cases, their repetition rate is quite high (e.g., hundreds of thousands of pulses per second).
Therefore, from the viewpoint of the instrument used to measure the signals associated with partial discharges, there is the problem of acquiring, very rapidly and efficiently, electrical signals having a high frequency content, preserving, as much as possible, the information content of the signals themselves. Moreover, said instrument should allow an effective separation of signals that are significant for the diagnosis from noise or from other “unwanted” signals.
The solution of the aforementioned problems is particularly difficult, considering the need to measure the partial discharges and to evaluate the state of the electrical system in unsupervised fashion, i.e. minimising the intervention of an operator (the operator, in certain cases, could be absent altogether, as is the case in line monitoring systems).
With regard to the state of the art in the sector instruments for measuring partial discharges (PD), the following is a list of known instruments, which can be subdivided in two categories: peak detectors (typically, narrow/selective band instruments) and software-controlled oscilloscopes (typically, wide band instruments). The main difference between peak detectors and controlled oscilloscopes is that peak detectors are not able to record the time dynamics of the detected pulse, because they do not perform an actual sampling of the signal, unlike oscilloscopes, which instead do perform said sampling. In this light, it should be noted that peak detectors are generally provided with a relatively narrow/selective band in order to be robust with respect to unwanted signals, in particular background noise.
Peak detectors are instruments that provide indications solely about the phase of occurrence and the amplitude of PD The amplitude of a PD is generally obtained passing the analogue signal through an FQI (quasi-integrator filter) and, hence, measuring the peak value. Within the field of peak detectors, the following types of instruments can be further distinguished:                Fully analogue instruments. The FQI is constituted by an analogue electrical grid. The output signal from the FQI is displayed on an oscilloscope. The available information is the known Lissajous figure, made available by the oscilloscope.        Mixed analogue/digital instruments. The FQI is still obtained by means of an analogue electrical grid and sent to a peak detector, also analogue, which provides the amplitude of the PD. The electronics of the instrument converts the amplitude signal of the PD (output by the peak detector) from analogue to digital. The available information is the set of the possible representations that can be obtained knowing phase and amplitude of the PDs, e.g., the known phase/amplitude pattern.        Fully digital instruments. The signal is filtered for the purpose of avoiding the known phenomenon of aliasing and, subsequently, it is sampled (analogue/digital transformation). The chain for calculating the amplitude of the PD (FQI filtering and detection of the peak of the signal output by the FQI) is obtained in this instruments by means of numeric algorithms. As in the previous case, the available information is the set of the possible representations that can be obtained knowing phase and amplitude of the PDs. It should be noted that said quasi-integrator filter FQI returns a signal whose information content (useful for a subsequent diagnosis of the electrical system to be evaluated) consists solely of a peak value of the wave form of the input signal, said value corresponding to the amplitude of the PD.        
Instruments are known from WO2005/038475A and U.S. Pat. No. 5,107,447A which enable the measurement of PDs and the judgement of predetermined features of those PDs, possibly using a real-time approach.
With regard to controlled oscilloscopes (controlled so they can acquire the signal output by the analogue circuits used to capture the partial discharge signal), there are known technical solutions developed in universities, for scientific research purposes. Known instruments have the following limits and disadvantages.
Peak detectors entail a drastic loss of information in the detected signals, due to the strong compression of the information associated with the partial discharge signal, due to the fact that they do not sample the detected signal and to the presence of the quasi-integrator filter. In particular, they do not allow to acquire significant information about the wave form of the pulses of the detected partial discharges. Therefore, they do not allow to separate (especially in automatic, unsupervised fashion) the noise from the discharge signals and, to a greater extent, the discharge signals coming from different sources. Moreover, peak detectors have a limitation in the phase of attributing the sign to the measured amplitude for a pulse, said sign having considerable importance, as is well known, in interpreting the results of the measurements (which are carried out, in most applications, subjecting the electrical components in question to alternating voltage). In particular, peak detectors do not allow to verify said attribution (possibly by performing the sign attribution step again, on the basis of a different criterion/calculation algorithm). With regard to the attribution of the sign of the discharges, it should also be noted that an additional limitation is associated with the instruments that detect pulses using a narrow band; a limited bandwidth could alter the time dynamic of the pulse, especially when the pulse has very rapid changes over time.
Therefore, in order to carry out a correct evaluation of the state of the insulation, the operator has to have a considerable experience, such as to allow him/her to evaluate the different contributions, separate them and provide and indication of their dangerousness. The procedure is complex in itself and it provides a subjective evaluation of the state of the insulation. To this should be added that, in many case, the operator's experience is anyway not sufficient to compensate for the loss of information during the signal detection step.
It should also be noted that instruments are known which, in addition to the amplitude of the pulse, are able to measure the time width of the pulse itself; such instruments are substantially peak detectors provided with a plurality of comparators (with different thresholds). Therefore, said instruments carry out, in fact, a sort of sampling (in certain case, with variable frequency). However, said instruments do not allow to detect the wave form of the pulse, i.e. the profile of the pulse amplitude over time, said wave form instead having fundamental importance for the purposes of a subsequent diagnostic evaluation of the acquired data.
On the contrary, oscilloscopes allow to measure discharge signals in wide band, and to acquire the entire wave form of the detected pulses. However, they are definitely costly and poorly reliable, especially in on-site applications, because they are particularly exposed to failures (e.g., in the presence of overvoltages). In fact, use of oscilloscopes is quite limited as far as field applications are concerned. Moreover, they are generally not able to reject noise by means of algorithms programmable within them. They also require the presence of a software residing in a computer, hence with the need to manage a large quantity of data.
Moreover, from the viewpoint of a diagnostic evaluation of the electrical system to be assessed, it can be useful (if not indispensable) to acquire, in addition to the partial discharge signals, also other quantities, e.g. quantities correlated to environmental factors (such as temperature and humidity), to be used in synergetic fashion together with data about partial discharge activity. Typically, said quantities undergo variations over time that are relatively slow, with respect to partial discharge signals; therefore, acquisition channels specifically dedicated to measuring said quantities are called “slow channels”. In this light, it should be noted that, in the oscilloscope/computer system, the need simultaneously to acquire through more than one channel and thus to proceed with a simultaneous detection of partial discharges and of said additional quantities entails considerable disadvantages, linked to calculation times and to the complexity of the software residing on the computer.
Therefore, the oscilloscope/computer system is penalising, not only from the cost viewpoint, but also from the performance viewpoint, because of the need to transfer enormous quantities of data from the oscilloscope to the computer.
With regard to the problems deriving from the need to transfer data from the oscilloscope to the computer, the following should also be noted.
The need to transfer a large quantity of data makes the whole data measurement/acquisition process very laborious. An additional problem is that the oscilloscope/PC communication is relatively slow and the storage memory is necessarily limited (this entailing a disadvantageous increase in dead time, i.e. of the time between the detection of one signal and the detection of the next signal), with the consequence that the oscilloscope/PC system is in distress when it has to measure in significant manner slow phenomena superposed to fast phenomena (i.e. signals with high repetition rates which are acquired simultaneously with signals with relatively low repetition rate). In practice, when it is necessary to measure a slow phenomenon (e.g., partial discharges that occur occasionally) simultaneously with a fast phenomenon (e.g. impulsive noise), the oscilloscope/PC allows only the two technical solutions that follow.                Measurement of a predetermined maximum number of signals (the maximum number allowed by the oscilloscope); in this way (in the best case), the slow phenomenon is also acquired, but there is the disadvantage of having to store an enormous quantity of data, with consequent slowing of all data measurement, transmission and, subsequently, processing operations.        Detection of a number of signals limited to a predetermined value; in this case, it is partially possible to speed up the process of measuring and subsequently processing the data, but there is the disadvantage of rapidly saturating the memory, with the risk of not acquiring the slow phenomenon in significant fashion.        
Therefore, the oscilloscope/computer system does not allow to measure different signals having mutually different time dynamics in efficient, optimised fashion, for the purposes of a diagnostic evaluation of an electrical system by analysing partial discharge signals.